1 . Technical Field
Embodiments of the subject matter disclosed herein generally relate to streamer steering devices that are attached to streamers or sources of marine seismic survey systems and are being used to control depth and/or lateral position of the streamer or source, and, in particular, to streamer steering devices having foldable wings.
2 . Discussion of the Background
Marine seismic surveys are used to generate an image of the geophysical structure under the seafloor in order to reduce the likelihood of drilling a dry well. During a seismic survey, as shown in FIG. 1, a vessel 110 tows one or more seismic sources 120 configured to generate acoustic waves 122a that propagate downward and penetrate the seafloor 124 until eventually being reflected by a reflecting structure 126.
Vessel 110 also tows acoustic detectors 112 arranged along a cable 114. Cable 114 and detectors 112 form what is known as a streamer 116. Detectors 112 acquire information (seismic data) about reflected waves 122b, 122c and 122d. 
Streamer 116 may be towed horizontally, i.e., lying at a constant depth relative to the water's surface 118 (as illustrated in FIG. 1) or slanted relative to surface 118 (as disclosed, for example, in U.S. Pat. No. 4,992,992).
Vessel 110 may tow plural streamers at the same time. During seismic data acquisition, the streamers' depth and lateral positions may be controlled using steering devices 130 known as “streamer steering devices.” The streamer steering devices are attached to the streamers.
A streamer steering device typically has solid control surfaces (referred to as wings or fins) attached to a body and allowed to rotate around an axis while an active area of the control surfaces is constant. When a streamer steering device's control surface moves through water, pressure on one side of such a surface may become greater than pressure on the other side thereof. The pressure difference yields a force perpendicular to the control surface and proportional to the area of its projection. This force is used to adjust a streamer steering device's location.
When the streamers are no longer used (i.e., the seismic survey has ended), they are retrieved on vessel 110. Conventional streamer steering devices, and, in particular, their control surfaces extending away from cable 114, may be damaged during recovery. In view of their length and flexibility, the streamers are usually retrieved and stored on spools located on the deck of the vessel. When the streamers are retrieved, the solid control surfaces of conventional streamer steering devices may be damaged or may get entangled, making it difficult to later roll out the streamers. Therefore, the wings of the streamer steering devices or the streamer steering devices themselves have to be removed from the streamers when the streamers are retrieved, which is a time-consuming procedure.
In instances when an accidental loss of towing speed occurs, the control surfaces may not accomplish the intended steering, but instead cause undesirable drag forces that amplify the slow-down. In other instances, if the streamers get too close to one another, the streamer steering device's control surfaces may become entangled with neighboring streamers. In still another instance, a wing of the streamer steering device may be damaged (e.g., broken) and thus, the streamer steering device may start to spin. In this situation, although the operator may be aware of the damaged streamer steering device, he or she can do nothing to minimize the disturbance created by its malfunction.
Furthermore, there are instances in which streamers need to be cleaned while deployed underwater to remove bio-fouling settlement that accumulates on their exterior surface. Cleaning devices used on the streamers are negatively impacted by the streamer steering device's control surfaces extending away from the streamer, because they prevent the cleaning devices from passing over the streamer steering device's location. Therefore, cleaning devices must be manually repositioned or they are limited to cleaning the streamer only between two adjacent streamer steering devices.
Accordingly, it would be desirable to provide streamer steering devices and methods that avoid the afore-described problems related to the control surfaces.